About this calculator
This calculator helps estimate how much paint a wall needs — enter the wall dimensions, the number of coats, and the paint coverage, and add any openings (doors, windows) separately so they are deducted from the paintable area. Useful for planning a room, corridor, or facade painting job.
Formula
The calculator converts the wall dimensions to whole millimetres, then:
gross area = length × height
openings area = Σ (opening width × opening height)
net area = gross area − openings area
paintable area = net area × number of coats
paint volume (excl. waste) = paintable area / coverage (m²/l)
waste volume = round(paint volume × waste% / 100)
total paint = paint volume + waste volume
containers required = ceil(total paint / container volume)
Openings are deducted as net area before multiplying by the number of coats — each opening is checked so that its sill height plus its own height cannot exceed the wall height (error if it does), and the total openings area cannot exceed the wall area (also an error). Paintable area (net area × coats) is exact geometry, independent of the coverage constant; only the volume calculation depends on the user-entered coverage rate (m²/l per coat). Container count always rounds up — 1.4 containers means 2 containers.
Worked example
Defaults: wall length 4 m, height 2.5 m, no openings, 2 coats, coverage 10 m²/l, waste 10%, container volume 10 l.
gross area = 4 m × 2.5 m = 10 m² (no openings, so net area is the same)
paintable area = 10 m² × 2 = 20 m²
paint volume (excl. waste) = 20 m² / 10 m²/l = 2.0 l
waste volume = 2.0 l × 10 / 100 = 0.2 l
total paint = 2.0 l + 0.2 l = 2.2 l
containers required = ceil(2.2 l / 10 l) = 1
For these inputs the calculator returns exactly this: paintable area 20 m², paint volume 2.2 l, and 1 container.
FAQ
Are openings (doors, windows) deducted from the paintable area? Yes. Every added opening is deducted from the wall area before computing how much area needs painting — this differs from the wallpaper calculator, where strip count doesn't deduct openings.
Why do I need to enter the number of coats, not just the area? Paintable area is computed as the net wall area multiplied by the number of coats — a second coat means the same area needs covering again, so the paint volume increases accordingly.
How much waste should be allowed for painting? The default is 10%, adjustable from 0 to 50. The real requirement depends on surface texture, the application tool (roller or spray), and losses during mixing and pouring.
What determines the paint coverage (m²/l)? Coverage is a value you enter, with a default of 10 m²/l — the real figure depends on the specific paint product and the surface's own porosity, so the calculator doesn't derive it from any standard.
Why does container count round up? Because paint can't be bought in fractions of a container — if the calculation needs 1.2 containers, you'll need to buy 2 full containers to have enough.
Assumptions and limits
No standard was found for paint coverage or area calculation — coverage depends on the
specific product, surface porosity, and application method, and the calculation itself is
plain geometry, not an engineering problem, so meta.standards is deliberately left empty.
Paint coverage (m²/l per coat) and container volume (litres) are user inputs with sensible defaults, not constants derived from a standard or manufacturer data.
The paintable-area calculation (net area × coats) is exact geometry; only the volume calculation uses the user-entered coverage rate. The drawing with openings is schematic — once there are more than six openings, the rest aren't drawn (but their area is still included), and the openings themselves are drawn spread evenly across the wall length, since the calculator doesn't collect their exact position in the wall.